Apparatus for and method of forming curved panels

ABSTRACT

Aluminium panels curved in mutually orthogonal x, y and z dimensions are made by stretch forming. A former ( 34 ) for use in this process comprises a plurality of steel bars ( 60 ) extending generally in the y direction and mutually spaced apart in the x direction. The bars ( 60 ) are held in side rails ( 62 ) and ( 64 ), which are formed with holes ( 66 ) to receive the bars ( 60 ), and flexed over an intermediate rail ( 68 ). Thus each bar ( 60 ) is curved in a yz plane. The holes ( 66 ) and the seats for the bars on the intermediate rail ( 68 ) each lie on a curve in an xz plane. So located, the bars ( 60 ) combine to define an envelope which is curved in mutually orthogonal x, y and z dimensions. When an aluminium sheet is stretched over the former ( 34 ), it takes up the shape of this envelope.

This invention concerns the forming of curved panels from sheet material, for the construction of reflectors for antennas including satellite communications antennas, and other purposes such as lighting reflectors and solar collectors.

A satellite communications antenna, for instance one used for TV signals, typically comprises a reflective parabolic dish several metres across arranged to collect an incoming signal and focus it at the focal point of the dish. Such dishes are typically constructed from a number of curved panels, and for effective performance each panel must be extremely accurate in form—typically an average error of not more than 0.5 mm over the panel. The panels are commonly made by means of a stretch forming machine in which a ductile aluminium sheet is stretched beyond its elastic limit (or “pulled”) over a former. The “skin” so formed is relatively delicate and therefore before use it is usually stiffened by “stringers” secured to the skin.

Heretofore the manufacture of antenna panels by stretch forming has been difficult and expensive for three reasons as follows. First, conventional stretch forming machines are designed principally for large-scale applications such as the manufacture of aircraft wings and fuselage sections and such machines are therefore needlessly massive (and hence unnecessarily costly) for the manufacture of antenna panels. For example, many sections typically manufactured by the stretch form process require considerable curvature in the panel, and therefore the conventional machine drives the former into the ductile sheet by means of a massive ram. The cost problem is exacerbated by the need for conventional stretch forming machines to be versatile, and therefore relatively complex, in order to accommodate a large variety of jobs and sustain the investment in the machine. Second, the former for such a machine is conventionally made from a solid block of material accurately machined to the required shape. Formers of this kind are very expensive to make, and more so to modify, which in turn means that non-standard reflectors cannot generally be afforded. It will also be understood that these formers are awkward to store. Third, the stringers supporting the panel are themselves conventionally pre-formed and secured to the skin by means of a special assembly fixture, adding two more cost elements to the manufacture of the antenna.

It is an object of the present invention to reduce the cost of manufacturing curved panels for use in the construction of satellite communications antennas and the like.

Thus according to a first aspect of the invention there is provided apparatus for forming from ductile sheet material a panel curved in mutually orthogonal x, y and z dimensions, which apparatus comprises two side rails and an intermediate rail between the two side rails, all said rails extending generally in the x direction, and a plurality of bars held by the side rails to extend generally in the y direction and be mutually spaced apart in the x direction, characterised in that the side rails and the intermediate rails are relatively configured and arranged so that the bars are flexed thereby to form a plurality of curves in yz planes and the apices of the curves in the yz planes form a curve in the xz plane.

There may be two or more said intermediate rails and there may also be two or more said side rails each side of these.

Each bar may be held at or adjacent its ends in openings formed in the side rails. The openings in the side rails may comprise holes for the respective bars each formed to extend along the curve in the yz plane of the bar it holds, the holes being mutually spaced apart in the y direction along a curve in the xz plane. Alternatively each opening may comprise a lengthwise slot holding a plurality of said bars.

Preferably each bar is flexed over a seat on the intermediate rail, and the seats mutually spaced apart in the y direction along a curve in the xz plane.

Those skilled in the art will appreciate that the use of multiple bars as a former is much cheaper to manufacture than the solid formers in general use heretofore, because instead of accurately machining a complete surface it is necessary only to provide accurate locations for the bars by means of the rails that hold them. Further, if a panel requires some modification, it is at most necessary to adjust the rails, which is much cheaper than reshaping a solid former. Also the assembly of rails and bars can be readily dismantled for convenient storage when not in use.

According to a second aspect of the invention there is provided apparatus for forming a panel from ductile sheet material, which apparatus comprises a former defining a surface curved in mutually orthogonal x, y and z dimensions and two clamps extending generally in the y direction and mutually spaced apart in the x direction to clamp said sheet material at opposite ends of said former, characterised in that said apparatus comprises a jack operable to drive said clamps apart and stretch the clamped sheet material over said surface.

Preferably each said clamp is pivoted about an axis extending generally in the y direction and rearward of said former (which is to say, in a direction opposite to the convexity of the former) whereby when the clamps are driven apart the sheet material is pulled rearwards onto said surface. The jack, which may be operable by fluid pressure, is preferably arranged between said clamps to bear on each thereof; or there may be two jacks, one on each clamp, bearing on each other.

This aspect of the invention takes advantage of the fact that items such as antenna panels have small curvatures in comparison with most stretch formed components such as aircraft parts. The action of the pivoting clamps draws the material down onto the former, as the material is strained, obviating the need for a massive ram to drive the former as heretofore. The pivoting arrangement of the clamps is a simple and therefore relatively inexpensive way of drawing the workpiece sheet onto the former. And by arranging one jack to act on both clamps, or two jacks to act against each other, there is no need for a particularly massive frame to resist the jacking force.

Each clamp may comprise first and second members arranged to receive said sheet material therebetween and a source of pressurised fluid arranged to drive the second member towards the second member thereby to clamp the sheet material, and there may be a flexible membrane between the second member and the source of pressurised fluid.

According to a third aspect of the invention there is provided a method of forming from ductile sheet material a panel curved in mutually orthogonal x, y and z dimensions, characterised in that said method comprises arranging a plurality of bars to extend generally in the y direction and be mutually spaced apart in the x direction, flexing the bars to form a plurality of curves in the yz plane, locating the bars so that the apices of the curves in the yz planes form a curve in the xz plane and stretching said sheet material over said bars.

A panel formed as aforesaid may be reinforced by stringers applied to the convex face of the panel while its concave face is on said former, and the stringers may be made from lengths of box section made flexible by kerfing before application to the panel. This means that the stringers are cheaply and easily made and their application to the panel does not require a special assembly fixture.

Panels included within the invention may be formed from aluminium and may be used in the construction of a communications antenna, or another product such as a lighting reflector or a solar heating reflector.

Other features of the invention will be apparent from the following description, which is made by way of example only and with reference to the accompanying schematic drawings, in which—

FIGS. 1 to 3 show in third angle projection three corresponding views of an antenna panel of the kind made by means of the invention, FIG. 1 being a front elevation, FIG. 2 being a side elevation and FIG. 3 being a plan view from below;

FIG. 4 is an isometric view of a stretch forming machine embodying the invention;

FIGS. 5 and 6 illustrate the operation of the stretch forming machine of FIG. 4;

FIG. 7 is an isometric view of a former for the stretch forming machine of FIG. 4;

FIG. 8 is a cross-sectional view of a clamp of the machine of FIG. 4;

FIG. 9 illustrates a stringer for application to an antenna panel;

FIG. 10 is a plan view of a former different from that shown in FIG. 7; and

FIG. 11 is a schematic section at A-A of FIG. 10.

Referring first to FIGS. 1 to 3, it should first be noted that the curvature of the antenna panel indicated at 10 therein is shown exaggerated for clarity of illustration. The panel 10 has a top 12 and a bottom 14 and two sides 16 and 18 and is curved so as to have a concave face 20 and a convex face 22.

The panel 10 is formed from aluminium alloy 1050A, which is ductile, and its concave face 20 acts as a focussing reflector for electromagnetic signals. Thus in use the panel 10 is put together with others (as indicated in broken lines at 24, 26 and 28) to construct a satellite communications antenna. The antenna is of parabolic form, but the details of its construction are not germane to the present invention, save that the concave face of each panel must have an average error of not more than 0.5 mm or possibly even less across its surface.

As can be seen (exaggerated) in FIGS. 1 to 3, and as will be understood by those skilled in the art, the panel 10 is curved in three mutually orthogonal dimensions, identified in FIGS. 1 to 3 and elsewhere herein as x, y and z.

FIG. 4 shows indicated generally at 30 a stretch forming machine according to the present invention.

A sheet 32 of aluminium is placed on a former 34 of the machine 30. The former 34, which will be described in more detail hereinafter, is configured and arranged to present a generally convex forward face (ie the upper face as seen in FIG. 4). This face is curved in mutually orthogonal x, y and z dimensions.

The sheet 32 is gripped at each end by a clamp 36 secured to a swinging frame 38 mounted on pivots 40 on the main frame 42 of the machine 30. The swinging frames 38 are each thus rotatable about an axis through their pivots 40, which are rearward of the convex face of the former 34 (ie below the former 34, as seen in FIG. 4). In use the swinging frames are driven by a hydraulic jack 50 (not shown in FIG. 4) as will now be described with reference to the diagrammatic FIGS. 5 and 6.

The hydraulic jack 50 is connected to each of the swinging frames 38. Before use, as shown in FIG. 5, the swinging frames 38 are positioned close up to the ends of the main frame 42 of the machine 30. In this position the aluminium sheet 32, held at each end by the clamps 36, rests on the convex face of the former 34. Pressurised fluid (from a source not shown) is admitted to the jack 50 which then drives the frames 38 apart, each rotating about its pivots 40. Thus the clamps 36 move arcuately and the sheet 50 is stretched and simultaneously pulled down onto the former 34, as shown in FIG. 6.

It should be noted that rotation of the swinging frames 38 and hence the stretching of the sheet 32 is exaggerated in FIGS. 5 and 6, for clarity of illustration, but in practice it is sufficient for the sheet 32 to be stretched beyond its elastic limit. In this way the sheet 32 is conformed to the shape of the convex face of the former 34. It should also be noted that each swinging frame 38 is cranked adjacent its pivots 40, as indicated at 38 a, which has the effect reducing the amount of rotation required to draw the sheet 32 onto the former 34.

Referring now to FIG. 7, the former 34 comprises a plurality of steel bars 60 extending in the y direction and mutually spaced apart in the x direction. The bars 60 are held in side rails 62 and 64, which are formed with holes 66 to receive the bars 60, and flexed over an intermediate rail 68. Thus each bar 60 is curved in a yz plane. Each hole 66 is bored to align with the flexure of the bar 60 it holds.

As can be seen from FIG. 7, the holes 66 lie on a curve in an xz plane. Similarly the seats (not detailed) for the bars on the intermediate rail 68 lie on a curve in an xz plane.

By these means the bars 60 combine to define an envelope which is curved in mutually orthogonal x, y and z dimensions. Therefore, when an aluminium sheet 32 is stretched over the former 34 as aforedescribed, it takes up the shape of this envelope. More particularly, the sheet 32 is given a convex face closely conforming to the envelope defined by the bars 60. It can then be released from the clamps 36 and trimmed (by means of a plasma cutter) along top, bottom and sides to provide an antenna panel as shown in FIGS. 1 to 3.

This arrangement is substantially cheaper than the conventional use of a solid former, even with a large number of bars 60. Further cost reductions can be effected by using fewer bars 60, but some consequent reduction in accuracy may limit the application of panels so made.

The accuracy of the panel (for any given application) is checked by means of a measurement system comprising commercial off-the-shelf (COTS) units that it is considered unnecessary to illustrate or describe. If the resulting shape is not exactly as required, the bars 60 can be removed and relocated, or the rails may be adjusted or new rails made.

After use the former 34 may be stored as an assembly, or alternatively dismantled for more compact storage. If it is dismantled the bars are removed and, being only elastically flexed, may then be used to make other formers. The rails 62, 64 and 68 may be retained for reuse in the event that a panel to the same design is required again.

FIG. 8 illustrates the construction of a clamp 36. An edge of the sheet 32 is located between a first member 70 and a second member 72. The first member is a fixed part of the clamp 36 and the second member 72 is a movable part thereof. The second member 74 is backed by a flexible membrane 74. Pressurised fluid 76 (from a source not shown, but which may be the same as that for the jack 50) is delivered behind the membrane 74 and thus the movable second member 72 is driven towards the edge of the sheet 32 which is then firmly held against the fixed first member 70.

The formed and trimmed panel 10 is reinforced by a stringer 80. The panel 10 is conveniently supported on the former 34 (or a separate similarly constructed assembly tool) during this process. The stringer 80 is manufactured from a length of straight box section material with a plurality of kerfs 82 cut through three sides thereof so as to be flexible. The side of the stringer 80 opposite the uncut side of the box section is placed on the panel 10 and conformed thereto by the application of weights or other means such as by air pressure. The stringer 80 is then secured to the panel 10 by any appropriate mean such as glue, rivets, bolts, screws or welding, or a combination of these means.

FIGS. 10 and 11 show a former 90 different from that described above with particular reference to FIG. 7. The former 90 has two side rails 92 and 94 and an intermediate rail 96 spaced apart in the y direction and extending generally (but not parallel) in the x direction. A plurality of steel bars 98 mutually spaced apart in the x direction extend between the side rails 92 and 94 and over the intermediate rail 96. Each of the bars 98 is flexed and held in place by pins 100 in the rails 92, 94 and 96. (For simplicity of illustration, only a few of the pins 100 are referenced in FIGS. 10 and 11. And for improved stability there may be pins 100 each side of each bar 98, at least in the side rails 92 and 94, although this is also not illustrated).

As shown in FIG. 11, the surface of the side rail 94 is curved in the xz plane and the other side rail 92 and the intermediate rail 96 are each similarly formed. By this means the bars 98, which are all similar and of rectangular cross section, are so located as to define an envelope (indicated in broken lines at 102 in FIG. 11) which is curved in the mutually orthogonal x, y and z dimensions. Thus, as aforedescribed in relation to the former 34, an aluminium sheet may be similarly stretched over the former 90, thereby to take up the shape of this envelope.

Various modifications and adaptations of the apparatus may be made. For instance, in the former 34 of FIG. 7, in place of the individual holes 66, the plurality of bars 60 may be held in lengthwise slots milled in the rails 62 and 64. In the former 90 of FIGS. 10 and 11, as an alternative to the arrangement described, the rails 92, 94 and 96 may each have a flat upper surface and the bars 98 be arcuately formed to define an appropriate envelope. The invention may use bars of any appropriate shape, not necessarily the circular cross section of the bars 60 or the rectangular cross section of the bars 98. Some material other than aluminium may be used for the panels. The single jack 50 shown in the drawings may be replaced by two jacks carried respectively by the two clamps and bearing upon each other. Further, panels may be made for applications other than satellite communications antennas, eg radar and microwave antennas, reflectors for high power lighting units of the kind used in sports stadiums and solar heating reflectors. Other possible modifications and adaptations will be apparent to those skilled in the art.

It will also be apparent that the tool described herein is of general utility, and can be used for methods other than stretch forming, for example in processes where the material is kept within its elastic limit and held in shape by strong stringers, or in the manufacture of items made from composite material.

Finally it should be noted that the x, y and z dimensions referred to herein and so annotated in the drawings are not intended to designate any specific orientation with respect to any forming machine or any former for such a machine or any panel made by such a machine. Thus, for instance, whilst FIG. 7 is described as showing a former with its bars 60 extending in the y direction (ie widthwise) and mutually spaced apart in the x direction (ie lengthwise), this is done purely for clarity of illustration. It should be understood that the invention does not call for an x dimension to be defined with reference to the length of any former or a y dimension to be defined with reference to its width. Similarly, with regard to FIG. 4, neither the lengthwise axis nor the widthwise axis of the machine shown therein defines a specific x or y dimension. And with regard to FIGS. 1 to 3, whilst the distance from head 12 to tail 14 of the panel 10 is shown aligned with the x, dimension, this does not mean that the x dimension is defined with reference to the head and tail of any panel 9 (and nor is the y dimension defined with reference to the sides of a panel). Rather, the x, y and z dimensions are simply conventional designations of three mutually orthogonal dimensions, and the panel 10, the former 34 and the machine 30 may be orientated in any way relative to those dimensions. It follows that the bars 60 may extend generally in the x direction as well as or instead of the y direction as described hereinbefore, and the bars may be flexed in an xz plane as well as or instead of a yz plane as described. Whilst the z dimension herein accords with the conventional reference to up-down dimensions, no such constraint is to be inferred. The x, y and z dimensions hereof can refer to any three mutually orthogonal dimensions. 

1. Apparatus for forming from sheet material a panel curved in mutually orthogonal x, y and z dimensions, which apparatus comprises: two side rails and an intermediate rail between the two side rails, all said rails extending generally in the x direction; and a plurality of bars held by the side rails to extend generally in the y direction and be mutually spaced apart in the x direction; wherein the side rails and the intermediate rails are relatively configured and arranged so that the bars are flexed thereby to form a plurality of curves in yz planes and the apices of the curves in the yz planes form a curve in the xz plane.
 2. Apparatus for forming a panel as claimed in claim 1 wherein the bars each have substantially the same cross section.
 3. Apparatus for forming a panel as claimed in claim 1 wherein each bar is held at or adjacent its ends in openings formed in the side rails.
 4. Apparatus for forming a panel as claimed in claim 3 wherein the openings in the side rails comprise holes for the respective bars each formed to extend along the curve in the yz plane of the bar it holds.
 5. Apparatus for forming a panel as claimed in claim 4 wherein said holes are mutually spaced apart in the y direction along a curve in the xz plane.
 6. Apparatus for forming a panel as claimed in claim 4 wherein each said opening comprises a lengthwise slot holding a plurality of said bars.
 7. Apparatus for forming a panel as claimed in claim 1 wherein each bar is flexed over a seat on the intermediate rail, said seats being mutually spaced apart in the y direction along a curve in the xz plane.
 8. Apparatus for forming a panel as claimed in claim 1 wherein there is more than one said intermediate rail.
 9. Apparatus for forming a panel as claimed in claim 8 wherein there is more than one said side rail each side of the intermediate rail or rails.
 10. Apparatus for forming a panel as claimed in claim 1, which apparatus comprises: a former defining a surface curved in mutually orthogonal x, y and z dimensions; two clamps extending generally in the y direction and mutually spaced apart in the x direction to clamp sheet material at opposite ends of said former; and a jack operable to drive said clamps apart and stretch the clamped sheet material over said surface; wherein one or each said clamp is carried on a frame pivoted about an axis extending generally in the y direction and rearward of said former, whereby when the clamps are driven apart the sheet material is pulled rearwards onto said surface.
 11. Apparatus for forming a panel as claimed in claim 10 wherein said jack is arranged between said clamps to bear on each thereof.
 12. Apparatus for forming a panel as claimed in claim 10 wherein each said clamp comprises first and second members arranged to receive said sheet material therebetween and a source of pressurised fluid arranged to drive the second member towards the second member thereby to clamp the sheet material, each said clamp including a flexible membrane between the second member and the source of pressurised fluid.
 13. A method of forming from sheet material a panel curved in mutually orthogonal x, y and z dimensions, wherein said method comprises: arranging a plurality of bars to extend generally in the y direction and be mutually spaced apart in the x direction; flexing the bars to form a plurality of curves in the yz plane; locating the bars so that the apices of the curves in the yz planes form a curve in the xz plane; and stretching said sheet material over said bars.
 14. A method of forming a panel as claimed in claim 13 wherein: the sheet material is clamped along edges extending in the y direction at opposite ends of said former and the clamped edges are driven apart to stretch the clamped sheet material over said surface; and wherein one or each said clamped edge is driven arcuately about an axis extending generally in the y direction and rearward of said former, whereby the clamped sheet material is pulled rearwards onto said surface.
 15. A method of forming a panel as claimed in claim 14 wherein the panel is reinforced by stringers applied to the convex face of the panel while its concave face is on said former.
 16. A method of forming a panel as claimed in claim 15 wherein each said stringer is made from a length of box section made flexible by kerfing before application to the panel. 17-19. (canceled) 